For an electrodeless high intensity discharge (HID) lamp, it is known that the principles of operation by which improved operating characteristics including high efficacy and good color rendering can be achieved, also yields the advantage that, in the absence of metal to glass seals as found in an electroded discharge lamp, the operating life of the discharge lamp can be greatly extended. U.S. Pat. No. 4,810,938 issued to Johnson et al on Mar. 7, 1989 and assigned to the same assignee as the present invention, describes an electrodeless HID lamp which is inductively driven by a high frequency RF current to produce a toroidally shaped arc discharge within an arc tube associated with the HID lamp. The arc tube can be ellipsoidally shaped and will contain a gas fill which comprises a combination of sodium halide and cerium halide along with xenon gas in proper weight proportions to generate the white color lamp emission which exhibits the improved efficacy and color rendering properties. U.S. Pat. No. 4,810,938 is hereby incorporated by reference.
One of the design considerations in improving the commercial practicability of the electrodeless HID lamp is the provision of a starting aid for assisting in initiating the arc discharge within the arc tube. An example of a starting aid for an electrodeless HID lamp can be found in U.S. Pat. No. 5,140,227 issued to Dakin et al on Aug. 18, 1992 and assigned to the same assignee as the present invention, such patent application being hereby incorporated by reference. In this patent application, it is disclosed that a starting aid stem, constructed of quartz, is formed in an extending manner from the arc tube. A gaseous fill contained within the starting aid stem is excited to an electric discharge state upon the introduction of a starting current thereto. The starting current is introduced to the starting aid stem by means of an electrical connection capacitively coupled to the upper end of the starting aid stem at a point beyond where the quartz starting aid stem exits an outer jacket associated with the electrodeless HID lamp. The outer jacket is formed around the arc tube entirely and also around a portion of the starting aid stem. Furthermore, the outer jacket is also constructed of quartz.
In disposing the outer jacket around the arc tube and a portion of the starting aid stem, one can achieve several benefits including: protecting against physical damage to the arc tube, protecting against surface contamination of the arc tube that could otherwise cause devitrification of the quartz material, and, preventing hydrogen, oxygen or water vapor from the ambient air from diffusing into the arc chamber thereby adversely affecting the gas mixture therein. One example of an electrodeless HID lamp which utilizes an outer jacket over the arc tube can be found in the previously cited U.S. Pat. No. 4,810,938. Though shown as having a protective outer jacket, such an approach is impractical for the electrodeless HID lamp which utilizes a starting aid stem extending from the arc tube. In such a construction, it is necessary to have a seal between the outer jacket and the portion of the starting aid stem which exits the outer jacket.
One way to seal between a quartz outer jacket and a quartz starting aid stem is by means of a flared seal. A flared seal is one in which an outwardly flared opening formed on the starting aid stem is formed to a sufficiently large dimension so as to extend over the edges of a cylindrically shaped outer jacket thereby allowing the overlap portion to serve as a sealing means to the space formed between the outer jacket and the arc tube. Such an arrangement has proven effective when the stem member need only be flared out to twice its normal diameter, however, when a factor of five flaring is needed as in the present instance, the stem member may be unnecessarily stressed by this flare seal process. An example of an electrodeless HID lamp which utilizes one type of a flared seal between the outer jacket and the starting aid stem can be found in U.S. Pat. No. 5,150,015 issued to R. A. Heindl et al on Sep. 22, 1992 and assigned to the same assignee as the present invention. With such a flared seal, it has been found that the process for implementing such a seal to an acceptable performance level in a mass production environment requires a time consuming and relatively difficult manufacturing process. For a product intended for mass production manufacturing techniques, it would be advantageous to utilize a sealing process that could be performed using automated equipment rather than the manual process used for the flared seal. Accordingly, though the flared seal arrangement is appropriate for a design stage of a lamp product such as an electrodeless HID lamp, when it is necessary to enter into a high volume production mode of operation, the flared seal approach is impractical.
An alternative to the flared seal arrangement can be found in a conventional electroded lighting product which utilizes a pinch seal to seal the arc tube around the electrode leads which extend through the ends of the arc tube. An example of a lamp using a pinch seal arrangement for sealing lamp lead-in wires within an end region of a lamp envelope can be found in U.S. Pat. No. 4,916,353 issued to Danko et al on Apr. 10, 1990 and assigned to the same assignee as the present invention. In this patent, an inner light transmissive cylinder is sealed within the lamp envelope thereby achieving a second glass or quartz structure within a lamp envelope. Though such a lamp illustrates one glass or quartz structure sealed within another, this lamp utilizes a metal lead-in wire to support the inner quartz cylinder and moreover, such inner cylinder is not joined to the lamp envelope at the point where the pinch seal is performed. With an electrodeless HID lamp, and particularly one which exhibits the quartz starting aid stem of the present invention, it is necessary to both seal the space within the outer jacket which surrounds the arc tube and to support the arc tube within the outer jacket. Furthermore, each of these functions must be accomplished without the benefit of the support that a lead-in wire arrangement can provide as in the instance of the electroded lamp. Of course, it is understood from the incorporated U.S. Pat. No. 4,810,938 that an electrodeless HID lamp provides the advantages of eliminating the glass to metal seals which are responsible for a shorter lamp life caused by the deficiency in seal integrity inherent in a glass to metal seal and that the electrodes are no longer involved in the discharge process.
Therefore, it would be advantageous to provide an electrodeless HID lamp having a starting tube extending from the arc chamber and an outer jacket which surrounds the arc chamber and at least a portion of the starting tube wherein the seal between the outer jacket and the extending starting tube constructed of the same materials achieved a high degree of seal integrity and could be implemented on a mass production manufacturing basis.
With respect to the process of sealing quartz material lamp products, it is customary to heat the quartz tube using oxygen-hydrogen or oxygen-natural gas fires, plasma flames or laser beams to a temperature at which the quartz becomes softened; the softened quartz is then pressed or pinched to form a hermetic seal at the end of the quartz tube. The typical application of such a process applies to a lamp product which utilizes a lead in wire. An example of a patent which addresses the problem of a leaky seal in the pinch seal region of the lead-in wire can be found in U.S. Pat. No. 3,868,528 issued to Lake et al on Feb. 25, 1975 and issued to the same assignee as the present invention. This patent discusses the fact that voids formed in the sealing process between an arc tube and a tungsten electrode can be filled using a glass sealant. This approach has proven effective in a lamp product having a tungsten electrode where, since the tungsten has a much greater coefficient of thermal expansion than the quartz material of the arc tube, crevices or gaps would inevitably form where the tungsten electrode extends into the quartz envelope without the use of the layer of sealing glass between the tungsten and quartz. Though this approach has proven effective for electroded discharge lamps, the step of coating the tungsten shank with a layer of glass sealant is not one available in the non-electroded quartz to quartz seal region associated with the electrodeless HID lamp of the present invention.